Various silica and silicate compounds have long been used as partial or complete replacements for carbon blacks in rubber compounding recipes. Their use is accompanied by increases in certain desirable physical properties such as tear resistance, hardness, etc. which may be of particular importance in the manufacture of tires and other articles. Use of silicas and silicates, however, also results in corresponding losses in other physical properties, particularly in modulus (stiffness). This has been attributed to the fact that, unlike carbon blacks, the silica compounds in and of themselves lack the ability to bond to the rubber molecular chains. It is this bonding which gives carbon black reinforced elastomers their superior viscoelastic properties.
To overcome the bonding deficiencies of silicas and silicates, there have been developed silica coupling agents which, through interaction between the silica compound and the rubber, provide a means for bonding the silicon compound to the rubber in a manner analogous to carbon black and effectively overcoming the inherent deficienies of using non-carbon black fillers.
The present invention provides for novel silica coupling agents, viz., certain substituted melamines, which may be used in both natural and synthetic rubbers and in blends thereof. The known silica coupling agents include bis-(3-[triethoxysilyl]propyl)-tetrasulfane manufactured by the Degussa Corporation. This compound is sold under the tradename of SI-69. Other silica coupling agents are described in an article entitled "Reinforcing Silicas and Silicates", M. P. Wagner, Rubber Chemistry and Technology, Volume 49, pages 730-737.
U.S. Pat. No. 3,775,366 to Wolff, et al, relates to a process for cross-linking natural or synthetic rubber having fillers, without the use of elemental sulfur. The sulfur is replaced by triazines which improve the workability of the rubber mixtures in the plastic state and which provide improved physical properties of the cross-linked rubber. The triazines of the invention are not used as silica coupling agents.
U.S. Pat. Nos. 3,801,537 and 3,923,724 to Westlinning, et al, relate to the use of substituted sim-triazinyl disulfide compounds to modify the reinforcing effects of a filler in a rubber vulcanizate. Neither of these patents teach the use of a substituted melamine of the present invention to act as a silica coupling agent.
U.S. Pat. No. 3,517,722 to Endter, et al, relates to the use of a methylene donor in conjunction with a methylene acceptor to form a resin in situ in a rubber compound, said resin improving the adhesion of metallic wire embedded in the rubber. While this patent encompasses the use of melamines, there is no suggestion of the use of the substituted melamines as silica coupling agents. Further, the substituted melamines in Endter, used as methylene donors must be reacted with the methylene acceptors, to produce a resin which is beyond the scope of the present invention.